Amplifier calibration

ABSTRACT

A system and method of calibrating an amplifier are presented. The amplifier has a first amplification path and a second amplification path. A first state of the amplifier is identified defining a first phase shift of the first path and a second phase shift of the second path resulting in a maximum efficiency of the amplifier when an attenuation of the first path and an attenuation of the second path are set to first attenuation values. The attenuation of the first path and the attenuation of the second path is set to achieve a maximum efficiency of the amplifier when the phase shift of the first path and the phase shift of the second path are set according to the first state.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/656,551 filed on Oct. 19, 2012 and entitled “AMPLIFIER CALIBRATION,”which is incorporated herein by reference.

FIELD OF THE INVENTION

Embodiments of the inventive subject matter relate to calibration of anamplifier in general and more specifically to techniques and apparatusfor calibration of a power splitter, where the power splitter includesan adjustable attenuation and phase state and operates in conjunctionwith a dual-path amplifier, such as a Doherty amplifier.

BACKGROUND OF THE INVENTION

Doherty amplifiers are amplifiers commonly used in wirelesscommunication systems. Today, for example, Doherty amplifiers are usedincreasingly in base stations that enable the operation of wirelesscommunications networks. Doherty amplifiers are suitable for use in suchapplications because the amplifiers include separate amplificationpaths, typically a carrier path and a peaking path. The two paths areconfigured to operate at different classes. More particularly, thecarrier amplification path typically operates in a class AB mode and thepeaking amplification path is biased such that it operates in a class Cmode. This enables improved power-added efficiency and linearity of theamplifier, as compared to a balanced amplifier, at the power levelscommonly encountered in wireless communications applications.

Generally, a power splitter supplies the input signals to eachamplification path in the Doherty amplifier. Power splitters or signalsplitters or dividers are known and used, as the name suggests, todivide or split a signal into two or more identical signals. When usedin conjunction with a Doherty amplifier, it is important that theattenuation and phase state of each path in the power splitter beproperly calibrated for a desired operation of the Doherty amplifier.Because each path can include a very large number of possibleattenuation and phase states, it is impractical to inspect the operationof the amplifier at each possible combination of attenuation and phasestates in order to identify the optimum calibration.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures, in which like reference numerals refer toidentical or functionally similar elements throughout the separate viewsand which together with the detailed description below are incorporatedin and form part of the specification, serve to further illustratevarious embodiments and to explain various principles and advantages allin accordance with the present inventive subject matter.

FIG. 1 is a block diagram showing a simplified representation of asystem for configuring an adjustable power splitter utilized inconjunction with a Doherty power amplifier system, in accordance with anembodiment.

FIG. 2 is a flowchart illustrating a method for calibrating theattenuation and phase state of a power splitter or an amplifier, inaccordance with an embodiment.

FIGS. 3A-3B are graphs showing a measured amplifier peak power as afunction of phase shift.

FIGS. 4A-4B are graphs showing a measured amplifier efficiency as afunction of phase shift.

FIGS. 5A-5B are graphs showing a measured amplifier efficiency as afunction of attenuation.

FIG. 6 is a graph showing a measured amplifier as a function of phaseshift at a number of distinct attenuation states.

DETAILED DESCRIPTION

In overview, the present disclosure describes embodiments of methods andsystems for calibration of an amplifier in general and more specificallytechniques and apparatus for calibration of a power splitter, where thepower splitter includes an adjustable attenuation state and phase state.As used herein, an “attenuation state” refers to a distinct combinationof attenuation levels applied to an input signal by attenuators alongmultiple amplification paths, where the attenuation levels fall within arange of attenuation levels supported by the system. The range ofattenuation levels may be divided into increments, where the attenuatorsmay be set to some or all of the incrementally different attenuationlevels in assessing performance of the amplifier (e.g., during a sweepthrough the attenuation states), as will be described later. Similarly,a “phase state” refers to a distinct combination of phase shifts appliedto an input signal by phase shifters along multiple amplification paths,where the phase shifts fall within a range of phase shifts supported bythe system. The range of phase shifts may be divided into increments,where the phase shifters may be set to some or all of the incrementallydifferent phase shift values in assessing performance of the amplifier(e.g., during a sweep through the phase shift states), as will bedescribed later. The power splitter is used in conjunction with adual-path amplifier, such as a Doherty power amplifier. Moreparticularly, various inventive concepts and principles embodied inmethods and apparatus corresponding to adjustable power splitterssuitable for use in amplifiers for improved efficiency and performance,and so on, will be discussed and disclosed. In the present disclosure,embodiments of the system are described in conjunction with a Dohertyamplifier, though it should be appreciated that in the presentdisclosure the Doherty amplifier may be replaced by an alternativedual-path amplifier, in other embodiments.

The instant disclosure is provided to further explain in an enablingfashion the best modes, at the time of the application, of making andusing various embodiments in accordance with the present invention. Thedisclosure is further offered to enhance an understanding andappreciation for the inventive principles and advantages thereof, ratherthan to limit in any manner the scope of the invention.

It is further understood that the use of relational terms, if any, suchas first and second, top and bottom, and the like are used solely todistinguish one entity or action from another without necessarilyrequiring or implying any actual such relationship or order between suchentities or actions.

Much of the inventive functionality and many of the inventive principlesare best implemented with or in integrated circuits (ICs) includingpossibly application specific ICs or ICs with integrated processing orcontrol or other structures. It is expected that one of ordinary skill,notwithstanding possibly significant effort and many design choicesmotivated by, for example, available time, current technology, andeconomic considerations, when guided by the concepts and principlesdisclosed herein will be readily capable of generating such ICs andstructures with minimal experimentation. Therefore, in the interest ofbrevity and minimization of any risk of obscuring the principles andconcepts according to below-described embodiments of the presentinvention, further discussion of such structures and ICs, if any, willbe limited to the essentials with respect to the principles and conceptsof the various embodiments.

Doherty amplifiers are used in a number of wireless applications, as theamplifiers enable high efficiency over a wide output power range and canachieve a desired linearity using various linearization schemes. In manyimplementations, Doherty amplifiers include two amplifiers, a firstcarrier (or main) amplifier, and a second peaking amplifier. In asymmetric Doherty amplifier, the carrier and peaking amplifiers are thesame size. Symmetric Doherty amplifiers are commonly used today, butasymmetric Doherty amplifiers that employ a peaking amplifier that islarger than the carrier amplifier offer the potential for additionalefficiency improvements.

In a Doherty amplifier, the input signal is split at an input or powersplitter between the carrier and peaking amplification path or circuit.The signals are then separately amplified by the carrier and peakingamplifiers of the Doherty amplifier and combined at an output stage.When combining the outputs of the carrier and peaking amplifiers, it maybe desired to make minor adjustments in the phase and amplitude orattenuation of the Doherty device's input splitter to provide optimalbalancing between the outputs of each path. To facilitate thisadjustment, a Doherty amplifier may include an adjustable power divideror splitter that can be used to fine-tune the configuration of the inputsignals to both the carrier and peaking amplifiers. The adjustable powersplitter allows both the attenuation and phase shift of each signalbeing transmitted to each path of the Doherty amplifier to be separatelyadjusted.

When using such an adjustable power divider, however, there are a largenumber of possible configurations for the attenuation and phase shiftsor phase states of each path. For example, in a conventional Dohertyamplifier having a carrier and a peaking path, each with an attenuatorhaving a state defined by four bits and a phase shifter having a statedefined by three bits, there are 128×128=16,384 possible states. Toproperly calibrate such a Doherty amplifier, the performance of theamplifier in each possible state may be tested. However, testing theamplifier performance in each of 16,384 states is very time consumingand inefficient. Additionally, due to the large number of possiblecombinations, it may be difficult to find an optimum attenuation andphase settings for the desired RF performance of the amplifier usingconventional techniques.

The present disclosure, therefore, describes embodiments that provide anapproach for calibrating a power splitter that may be used inconjunction with a Doherty amplifier. The various embodiments includecalibrating the attenuation and phase states of both the main andpeaking paths of the amplifier, so that each path is optimally matched.Moreover, the various embodiments may greatly reduce the number ofcombinations between attenuation and phase states in each amplificationpath that may be tested before calibration. The embodiments also mayallow further improvement in efficiency as a function of the usedlinearization quality. As such, the efficiency of the amplifier may bemaximized to an extent where the linearization system can correct thenonlinearity of the amplifier, according to various embodiments.

FIG. 1 shows a simplified and representative high level diagram of anembodiment of a system for configuring an adjustable power splitterutilized in conjunction with a Doherty power amplifier system. In FIG. 1as shown, an adjustable power splitter 101 or radio frequency powersplitter is coupled to or utilized with an amplifier, specifically aDoherty power amplifier 103. The adjustable power splitter 101essentially divides an input signal into multiple amplification paths,where each amplification path includes an adjustable attenuator (e.g.,one of attenuators 115, 121), an adjustable phase shifter (e.g., one ofphase shifters 113, 119), and an amplifier (e.g., one of amplifiers 135,139).

The adjustable power splitter 101 includes a power divider 105 with aninput 107 for receiving an input radio frequency signal (RFIN), andfirst and second divider outputs 109, 111, respectively. In asymmetrical Doherty amplifier, the power divider 105 of FIG. 1 operatesto divide or split an input signal received at the input 107 into twosignals that are very similar with, in some embodiments, equal power.This equal power form of power divider may be referred to as a 3 decibel(dB) divider when the resultant signals are each 3 dB less than thesignal at the input. While a 3 dB divider is typical, other dividerswith multiple outputs or outputs with unequal signals could be fashionedand used in some applications, in other embodiments.

Further included in the adjustable radio frequency power splitter 101,as shown in FIG. 1, is a first adjustable phase shifter 113 and a firstadjustable attenuator 115, which are series-coupled to the first divideroutput 109 and configured for providing a first power output 117. Itwill be appreciated that the adjustable phase shifter and adjustableattenuator can be series-coupled to each other in any order (e.g.,attenuator followed by phase shifter as shown or vice versa). Furtherincluded in the adjustable radio frequency power splitter 101 is asecond adjustable phase shifter 119 and in some embodiments a secondadjustable attenuator 121, which are series-coupled to the seconddivider output 111 and configured for providing a second power output123. As noted above, the order in which these components areseries-coupled to each other can be changed.

In various embodiments of the adjustable power splitter 101, the firstand typically the second adjustable phase shifter 113, 119 are eachdigitally controlled (e.g., by a controller 125) and have a plurality ofstates. In one or more embodiments, the first adjustable phase shifter113 and often the second adjustable phase shifter 119, each have eightphase shifted states where each phase shifted state defines a particularphase shift in degrees, although they may have more or fewer phaseshifted states, in other embodiments. In one example, the phase shiftedstates are separated by approximately 6.5 degrees. It will beappreciated that the first and second adjustable phase shifters 113, 119may have different phase shifted states, cover different ranges, andhave different steps sizes from one another, although typically theywill be essentially the same. While digitally controlled, the adjustablephase shifters in many embodiments are analog phase shifters.

In various embodiments of the adjustable power splitter 101, the firstand typically the second adjustable attenuator 115, 121 are eachdigitally controlled (e.g., by controller 125) and have a plurality ofattenuation levels where the attenuation levels are separated by anumber of dB (in one 0.5 dB separates the attenuation levels). In one ormore embodiments, the first adjustable attenuator 115 and often thesecond adjustable attenuator 121, each have eight attenuation states orattenuation levels, although they may have more or fewer attenuationstates or attenuation levels, in other embodiments. It will beappreciated that the first and second attenuation may have differentattenuation states, cover different attenuation ranges, and havedifferent attenuation steps sizes from one another, although typicallythey will be essentially the same. While digitally controlled, theadjustable attenuators in many embodiments are analog attenuators.

Some embodiments of the adjustable power splitter 101 further include anoptional fixed phase shifter 127 that is configured for adding a fixedphase shift between first and second signals at the, respectively, firstand second power outputs 117, 123. In some embodiments, this can be afixed and predetermined phase shift (e.g., 90 degrees) added to oneamplification path (e.g., the amplification path between output 109 andpower output 117, or the amplification path between output 111 and poweroutput 123).

In certain applications (e.g., within Doherty amplifier 103), a ninetydegree phase shift is added to one path in the amplifier, and the fixedphase shift can be used to offset this amplifier phase shift. The fixedphase shift in some embodiments includes a phase shift in a negative orpositive direction (e.g., a negative shift λ/8 129, such as a negativeforty five degree shift) for the first signal at the first power output117, and a phase shift in the opposite direction (e.g., a positive shiftλ/8 131 such as a positive forty five degree phase shift) for the secondsignal at the second power output 123. Using the opposite-signed, fortyfive degree phase shifts yields a ninety degree relative phase shiftbetween the signals at the power outputs 117, 123. The phase shifter 127(or negative shift 129 and positive shift 131) can be lumped elementcircuits having an inductive and a capacitive reactance, according to anembodiment, as will be further discussed below with reference to FIG. 2.

The controller 125 is configured and arranged for controlling theadjustable phase shifters 113, 119 and adjustable attenuators 115, 121of the adjustable power splitter 101. For example, controller 125 may beconfigured to calibrate the adjustable power splitter 101 in accordancewith the methods described in the present disclosure. For datacommunications, the controller 125 includes an input 133 (orinput/output) that is coupled to a data interface (e.g., a serialinterface such as a serial peripheral interface (SPI), not illustrated).The data interface (e.g., the SPI) may be implemented on the sameintegrated circuit chip as the power splitter 101 (e.g., a singlesilicon or gallium-arsenide chip), or the data interface and the powersplitter 101 may be implemented on different integrated circuit chips(e.g., two silicon chips, two gallium-arsenide chips, or a combinationof one silicon chip (e.g., for the SPI) and one gallium-arsenide chip(e.g., for the power splitter 101)).

Generally, the attenuators 115, 121 and/or phase shifters 113, 119 arecontrolled using a number of switches, typically solid state orintegrated switches such as those implemented as transistors. Thus, thecontroller 125 can be provided state information for all switches in allattenuators 115, 121 and phase shifters 113, 119, and the controller 125essentially acts as one or more latching buffers with outputs arrangedand coupled to ensure that all switches are in the appropriate ON or OFFstate. Alternatively, the controller 125 can be provided an encodedvalue (e.g., a binary value) or two or more encoded values, wherein eachof the encoded values uniquely specify a state for each attenuator 115,121 and phase shifter 113, 119. For example, if all phase shifters 113,119 and attenuators 115, 121 are 8 state devices, a 3 bit encoded valuefor each could be used to uniquely specify a particular state.Accordingly, during operation, 4 such encoded values could be providedto the controller 125 (e.g., one for each attenuator 115, 121, and onefor each phase shifter 113, 119). The controller 125 may then converteach encoded value to the appropriate control signals (e.g., switchcontrol signals) for each attenuator 115, 121 and phase shifter 113,119, and latch in these values. In other embodiments, the amount ofphase shift and attenuation for each of the four devices 113, 115, 119,121 could be sent to the controller 125, and the controller 125 coulddetermine the proper state to realize the desired shifts andattenuations. In another alternate embodiment, the controller 125 mayreceive an address or offset, and may look up the phase state and/orattenuator state information in a lookup table (not illustrated) basedon the received address or offset.

The Doherty amplifier 103 includes a main or carrier amplifier 135coupled via a matching network or circuit (not illustrated) to the firstpower output 117 and a peaking amplifier 139 coupled by via matchingnetwork or circuit (not illustrated) to the second power output 123. Aswill be appreciated by those of ordinary skill based on the descriptionherein, the main and peaking amplifiers 135, 139 may be comprised of oneor more stages of relatively low power level amplification andrelatively high power level amplification.

The carrier and peaking amplifiers 135, 139 are coupled via respectiveoutput matching circuits (not illustrated) to a Doherty combiner 147,which is configured such that the carrier amplifier 135 provides theamplification for lower level signals, and both amplifiers 135, 139operate in combination to provide the amplification for high levelsignals. This may be accomplished, for example, by biasing the carrieramplifier 135, such that it operates in a class AB mode, and biasing thepeaking amplifier 139 such that it operates in a class C mode. Morecomplex embodiments are possible in which the adjustable power splitter101 has three outputs, and the Doherty amplifier 103 has a main and twopeaking amplifiers, with each peaking amplifier biased in differentclass C operating points. In such an embodiment, the power splitter 101may include three amplification paths (each including an adjustablephase shifter and an adjustable attenuator). In one or more of thesemanners, overall efficiency/linearity of the amplifier can be improvedover a relatively wide range of signal levels. Adjustments to theadjustable attenuators 115, 121 and adjustable phase shifters 113, 119can be made in an experimental manner by monitoring power drawn by thepeaking stage 139 or main stage 135 or both as a function of inputsignal levels and the like. At certain input signal levels, the peakingamplifier 139 should begin to operate, and amplitude and phaseadjustments can be made with this in mind.

As shown in FIG. 1, and according to an embodiment, current meters 149and 151 are connected, respectively, to the current conducting terminals(e.g., drains) of carrier amplifier 135 and peaking amplifier 139.Current meters 149 and 151 are configured to measure a current flowthrough each path of the Doherty amplifier 103. A power meter 153 isalso connected to the output of the Doherty amplifier 103, in anembodiment. The power meter 153 is configured to measure both averagepower generated by the Doherty amplifier 103 as well as apeak-to-average power ratio (PAR) of the amplifier 103, or, as analternative, measure only the peak power generated by the amplifier.During calibration of the adjustable power splitter 101, measurementsproduced by current meters 149 and 151 and power meter 153 are used inidentifying an optimized configuration of each of the adjustableattenuators 115, 121 and the adjustable phase shifters 113, 119.

In some embodiments of the present system, the controller 125 isconfigured to execute an algorithm for calibrating the adjustable powersplitter 101. In that case, the controller 125 can be placed incommunication with each of current meters 149, 151 as well as powermeter 153 to receive data therefrom. The data collected from the meters149, 151 can then be used in calibrating the adjustable power splitter101 for a desired operation.

To achieve a desired performance of the amplifier shown in FIG. 1, thepower splitter 101, and, specifically, the attenuation and phase statesof the power splitter 101, must be correctly calibrated. As used herein,“calibration” refers to setting the attenuation and phase shifters ofthe power splitter 101 to different attenuation levels and phase shifts,respectively, to determine a configuration of the attenuation levels andphase shifts that results in an optimal or near-optimal performance.Because each of the adjustable attenuators 115, 121 and adjustable phaseshifters 113, 119 may have a large number of candidate states, it can bedifficult to determine which of those large number of candidate statesmay achieve the desired amplifier performance.

Embodiments discussed in the present disclosure, therefore, provideapproaches for efficiently calibrating a power splitter coupled to andused in conjunction with a Doherty amplifier. The various embodimentsallow for determining the optimal or near optimal attenuation and phasevalues along the carrier and peaking paths of a Doherty amplifier fromamong a potentially large number of possible phase and attenuationstates. The attenuation and phase values are selected to provideefficient amplifier operation that still meets the amplifier'slinearization requirements, in an embodiment. As will be described inmore detail below, the embodiments efficiently locate the amplifier'soptimal or near optimal state, while minimizing or reducing the numberof iterations. Furthermore, the embodiments can be used to identify theoptimal or near optimal values for attenuation and phase state dependingon the desired performance characteristics.

FIG. 2 is a flowchart illustrating a method for calibrating theattenuation and phase states of a Doherty amplifier (or a power splitterconnected thereto) in accordance with an embodiment. The method may beimplemented, for example, by controller 125 of FIG. 1. Alternatively,the method can be implemented by any device component or entity havingthe ability to control the attenuation and phase states of theamplifier's carrier and peaking amplification paths, while alsomonitoring the output of the amplifier, such as a host computer incommunication with adjustable attenuators and phase shifters (e.g.,attenuators 115, 121 and phase shifters 113, 119, FIG. 1). For example,the method can be implemented using electronic circuit monitoringdevices.

In the case of a symmetrical Doherty amplifier, in step 201 theattenuation of both the carrier and the peaking amplification paths ofthe amplifier typically are set to be equal to one another. In somecases, this may involve setting the attenuation level applied along eachpath to 0 dB. With reference to the device of FIG. 1, this may involvesetting the attenuation of each of adjustable attenuators 115, 121 ofFIG. 1 equal to 0 dB. However in an asymmetrical amplifier, theattenuation of both the carrier and the peaking amplification paths maybe set to different values, which correspond to the power ratio betweencarrier and peaking amplifiers.

With the attenuation level (or state) of each path set in step 201, aninput signal (e.g., an RF signal) is supplied to the amplifier in step203. The input signal may be selected to mimic the input signals thatwill be fed into the amplifier during normal use. In one embodiment, forexample, the input signal mimics a digitally modulated signal commonlyencountered in wireless communication applications. In other cases, theinput signal may include an arbitrary waveform supplied to the powersplitter (e.g., power splitter 101) that drives the amplifiersufficiently to draw current in both the carrier and peaking paths. Inone embodiment, the input signal is selected to have sufficient power toachieve a 6-7 dB output back-off.

With the input signal being supplied to the amplifier, in step 205 thephase shifts of the carrier and peaking paths of the Doherty amplifierare swept through a number of possible combinations (e.g., the phaseshifters 113, 119 are controlled by controller 125 to apply differentcombinations of phase shifts to the input signal while the attenuationapplied by attenuators 115, 121 is held constant, and the output signalis measured for each combination). Each combination of phase shifts ofthe carrier and peaking paths may be referred to as a phase state of theamplifier or the power splitter. In one embodiment, step 205 involvessweeping the carrier and peaking paths through all possible phase shiftcombinations or phase states. In other embodiments, only a subset ofavailable phase states that surround a nominal, or default, phase shift(e.g., in a conventional Doherty amplifier the nominal phase shift is 90degrees) between paths are swept. For example, if a relative phase sweepof fewer than 180 degrees is considered sufficient (e.g., because such asweep is considered to cover a sufficient number of different phasestates around 90 degrees), it may be sufficient to consider only asubset of the available phase states.

One embodiment for testing combinations of phase states for the deviceof FIG. 1, for example, may be as follows. The first adjustable phaseshifter 113 may be configured to apply a first phase shift (e.g., inresponse to controller 125 receiving an encoded value corresponding to afirst phase shift, such as binary value 000). Then, the secondadjustable phase shifter 119 is swept through all of its possible phaseshifts (e.g., by cycling through phase shifts corresponding to binaryvalues 111 to 000 (from maximum phase to minimum phase), or some subsetthereof). During the sweep, each phase state is considered a “candidatephase state.” Once the sweep is complete, the second adjustable phaseshifter 119 is set to a second phase shift (e.g., a phase shiftcorresponding to binary value 000). Then, the first adjustable phaseshifter 113 is swept through all of its possible phase shifts (e.g., bycycling through phase shifts corresponding to binary values 001 to 111,or some subset thereof). Again, during the sweep, each phase state isconsidered a “candidate phase state.” As such, the phase shifts appliedby the first and the second adjustable phase shifters are swept around adesired phase shift range (e.g., around 90 degrees).

During the execution of step 205, when the first and second adjustablephase shifters 113, 119 are set in each candidate phase state, thecurrent (e.g., drain current) through each path in the amplifier, andthe output power and PAR of the output signal from the amplifier aremeasured. In one embodiment, the output power and PAR of the outputsignal are captured by power meter 153, while the currents of each pathare captured by current meters 149 and 151. Having captured that data,the amplifier's efficiency can be calculated for each phase state usingthe output power and the total current values. In one examplecalculation, the amplifier's efficiency equals Pout/Pdiss; where Poutequals measured output power and Pdiss (the dissipated power on theamplifier) equals Vd*I, where Vd equals drain voltage, and I equalstotal measured drain current.

Then, having captured the output power, drain currents of each path, PARof the output signal, and amplifier efficiency for each candidate phasestate, a pair of phase states are identified and stored in step 207.First, the phase state of the Doherty amplifier that resulted in themaximum peak output power is stored as State 1. State 1, therefore, isdefined by the phase shift of both the first and second adjustable phaseshifters that resulted in the amplifier having the maximum peak outputpower with zero attenuation on each path. Second, a phase state of theDoherty amplifier that resulted in the most efficient output of theamplifier (or maximum efficiency) is saved as state State 2. State 2,therefore, defines the phase shifts of both the first and secondadjustable phase shifters that resulted in the amplifier having the mostefficient output with zero attenuation on each path. Each of States 1and 2, therefore, define particular attenuation and phase calibrationsof the amplifier for various performance criteria.

FIGS. 3A, 4A, and 5A show experimental data for a nominal amplifierdevice. In a nominal device the components of the amplifier (and,specifically, the transistors making-up each amplifier path) arewell-balanced against one another allowing for efficient operation ofthe amplifier, thus providing good efficiency, linearity and peak power.Conversely, FIGS. 3B, 4B, 5B show experimental data for a devicecomprising a number of poorly matched components. In a non-nominaldevice, the transistors making up each amplifier path may exhibit themaximum manufacturing variation, resulting in poor amplifierperformance. As such, the figures show a somewhat worst-case scenario.FIGS. 3A-3B and 4A-4B are graphs illustrating sample data that may becaptured during the execution of steps 205 and 207 of the methoddepicted in FIG. 2.

FIGS. 3A and 3B each depict a measured amplifier peak power as afunction of phase state (e.g., indicating phase shift between thecarrier and peaking paths of the amplifier). The measurement of peakpower can be used to provide information regarding the amplifier'slinearity, as the amplifier's linearity is proportional to the measuredpeak power. As such, a high peak power indicates good linearity. In eachset of test results, pursuant to step 205 of FIG. 2, the phase state ofthe amplifier has been swept through a number of potential phase shiftsabout the nominal 90 degree phase shift between the carrier and peakingpaths, while the attenuation of each path has been fixed. At eachcandidate phase state, the amplifier peak power has been measured. Then,pursuant to step 207 of FIG. 2, the phase state giving rise to thehighest peak power has been identified. This point defines State 1,described above. In FIG. 3A, for the nominal amplifier, point 301indicates the peak power of the amplifier at a phase shift between thecarrier and peaking paths of approximately 96.5 degrees. Similarly, inFIG. 3B, for the non-nominal amplifier, point 303 indicates the peakpower of the amplifier at a phase shift between the carrier and peakingpaths of approximately 77 degrees.

FIGS. 4A and 4B each depict measured amplifier efficiency as a functionof phase state. In each set of test results, pursuant to step 205 ofFIG. 2, the phase state of the amplifier has been swept through a numberof potential phase states about the nominal 90 degree phase shiftbetween the carrier and peaking paths. At each candidate phase state,the amplifier efficiency has been evaluated. Then, pursuant to step 207of FIG. 2, the phase state giving rise to the highest efficiency hasbeen identified. This point defines State 2, described above. In FIG.4A, for the nominal amplifier, point 401 indicates the maximumefficiency of the amplifier at a phase shift between the carrier andpeaking paths of approximately 135.5 degrees. Similarly, in FIG. 3B, forthe non-nominal amplifier, point 403 indicates the peak power of theamplifier at a phase shift between the carrier and peaking paths ofapproximately 83.5 degrees.

Returning to the method of FIG. 2, with the most efficient phase stateand maximum peak output power phase state determined, the adjustablephase shifters of the power splitter of the Doherty amplifier areconfigured in accordance with State 2 (maximum efficiency) in step 209.In an alternate embodiment, the adjustable phase shifters may beconfigured in accordance with a state corresponding to some otherperformance criteria. Then, in step 211, with the phase shifts of thecarrier and peaking paths invariant, the attenuation states of theamplifier's carrier and peaking paths are swept. Depending upon theimplementation, this may involve sweeping only the attenuation levelsapplied along a single path (e.g., the peaking path). In otherimplementations, all possible attenuation states including all possiblecombinations of attenuation levels of the carrier path in combinationwith the peaking path are swept.

With reference to FIG. 1, for example, when sweeping only a single path(e.g., the peaking path), if adjustable attenuator 115 has a number ofdifferent attenuation levels corresponding to encoded binary valueshaving 4 bits, only the 15 possible attenuation levels are swept, orsome subset thereof. In that case, when sweeping the candidateattenuation levels of the peaking path, the attenuation of the carrierpath is set to 0 dB allowing for optimum efficiency of the amplifier.This approach results in fewer data points to analyze (e.g., the numberof data points will be equal to the number of potential states of theadjustable attenuator in the peaking path), resulting in a moreefficient analysis.

However, when sweeping the attenuation states corresponding to allpossible attenuation levels on both paths, if each of adjustableattenuators 115 and 121 has attenuation levels defined by 4 bits, theneach attenuator has 16 possible attenuation levels. In that case, theadjustable attenuators of the carrier and peaking paths, in combination,have a total of 256 possible attenuation levels. In step 211, theadjustable attenuators are swept through each one of those attenuationstates. In an alternate embodiment, the adjustable attenuators may beswept through a subset of all possible attenuation states.

One embodiment for testing each candidate attenuation state may be asfollows. The first adjustable attenuator 115 may be configured to afirst attenuation level (e.g., in response to controller 125 receivingan encoded value corresponding to a first attenuation level defined bybinary value 0000). Then, the second adjustable attenuator 121 could beswept through all of its possible attenuation levels (e.g., by cyclingthrough attenuation levels corresponding to binary values 0000 to 1111,or some subset thereof). During the sweep, each combination ofattenuation levels of the first adjustable attenuator 115 and the secondadjustable attenuator 121 is considered a “candidate attenuation state.”Once the sweep is complete, the first adjustable attenuator 115 is setto a second attenuation level (e.g., an attenuation level correspondingto binary value 0001). Then, the second adjustable attenuator 121 isswept through all of its possible attenuation levels (e.g., by cyclingthrough attenuation levels corresponding to binary values 0000 to 1111,or some subset thereof). Again, during the sweep, each combination ofattenuation levels of the first adjustable attenuator 115 and the secondadjustable attenuator 121 is considered a candidate attenuation state.This process could be repeated until the first adjustable attenuator 115reaches its final attenuation level (e.g., an attenuation levelcorresponding to binary value 1111).

During the execution of step 211, when the first and second adjustableattenuators shifters 115, 121 are set in each candidate attenuationstate, the current (e.g., drain current) through each path in theamplifier, and the output power and PAR of the output signal from theamplifier are measured. In one embodiment, the output power and PAR ofthe output signal are captured by power meter 153, while the currents ofeach path are captured by current meters 149 and 151. Having capturedthat data, the amplifier's efficiency can be calculated for eachattenuation state using the output power and the total current values.Then, pursuant to step 213 of FIG. 2, a third state (State 3) isidentified in step 213 that results in the amplifier maximum efficiency.State 3, therefore, is defined by the attenuation level of both thefirst and second adjustable attenuators that resulted in the amplifierhaving the maximum efficiency, where the phase state of each path in theamplifier has been fixed according to State 2. During the attenuationsweep, in one embodiment, the swept attenuation range may be limited tohalf (or some other percentage) of the available ranges of eachattenuator.

FIGS. 5A-5B are graphs illustrating sample data that may be capturedduring the execution of steps 211 and 213 of the method depicted in FIG.2. FIGS. 5A and 5B each depict measured amplifier efficiency as afunction of attenuation states. In each Figure, the X-axis representsattenuation, where negative values represent attenuation of the carrierpath, while positive values represent attenuation of the peaking path.In each set of test results, pursuant to step 211 of FIG. 2, the phasestate of each path of the amplifier has been held constant in accordancewith State 2, as described above. Then, the attenuation state of theamplifier has been swept through a number of candidate attenuationstates.

In the examples shown in both of FIGS. 5A and 5B a negative value on theX-axis indicates that the attenuation of the carrier path is set to Y dB(e.g., when the attenuation of the peaking path is set to 0 dB).Conversely, a positive value on the X-axis indicates that theattenuation of the peaking path is set to Y dB (e.g., when theattenuation of the carrier path is set to 0 dB).

In FIG. 5A, for the nominal amplifier, point 501 indicates the peakefficiency of the amplifier at half of the range of attenuation on thepeaking path and an attenuation of 0 dB on the carrier path at a phaseshift of 135.5 degrees between paths. In FIG. 5B, for the non-nominalamplifier, point 503 indicates the peak efficiency of the amplifier atan attenuation of half of the range of attenuation on the peaking pathand an attenuation of 0 dB at a phase shift of 83.5 degrees betweenpaths.

Returning to FIG. 2, the three states, State 1, State 2, and State 3defined in steps 207 and 213 form the boundary values for theamplifier's attenuation and phase states giving rise to the optimalamplifier configuration. Within those boundary conditions, however, asuitable amplifier configuration may be defined. The suitableconfiguration may depend, for example, upon the linearization systemimplemented within the amplifier and the peak output power clipping ofthe amplifier, among other things.

In general, it is desirable to configure the amplifier in the mostefficient state that still provides acceptable linearity. Therefore, aslong as the linearization system of amplifier can correct thenonlinearity of the amplifier within the measured three states, theattenuator and phase shifter states are tuned towards a most efficientstate (e.g., State 3). On the contrary, if the linearization system isnot able to correct nonlinearity, the attenuator and phase shifterstates are shifted towards a state having improved linearity over thelinearity associated with that of the amplifier when turned towardsState 3.

Accordingly, having identified State 3 in step 213, in step 215 theamplifier is configured in accordance with the state defined by State 3.As such, the adjustable phase shifters of the carrier and peaking pathsare configured so as to produce the maximum efficiency from theamplifier when both the carrier and peaking paths have the sameattenuation, while the adjustable attenuators of both the carrier andpeaking path are set to achieve the maximum efficiency given such aphase state, in an embodiment. Then, in step 217, a determination as towhether the current state of the amplifier is linearizable is made. Inone embodiment, “linearizable” means that the adjacent channel powerratio (ACPR)) of the amplifier is within an acceptable range. Oneexample range includes the 3GPP Spectrum Emissions Mask for the givensignal type. The determination of whether the amplifier is linearizablecan be made utilizing the amplifier's linearization system (e.g. digitalpredistortion). After linearization is performed, if the linearity ofthe amplifier is within an acceptable range, then the amplifier isconsidered to be linearizable. The 3GPP Spectrum Emissions Mask is aspecification that includes performance parameters that should be met bybase stations. If the amplifier is determined to be linearizable, asuitable configuration for the amplifier has been identified and themethod ends in step 219 with the amplifier being configured inaccordance with State 3.

If, however, the current state of the amplifier is not linearizable, instep 221 the state of the amplifier is transitioned towards the statedefined by State 1. This adjustment may be made according to a number ofdifferent algorithms. In one example, in step 221 the amplifier's phasestate and attenuation state are each adjusted by one increment towardstheir values in State 1. Alternatively, in step 221 either theattenuation or the phase state may be adjusted towards State 1, whilethe other state is not adjusted.

After adjusting the state of the amplifier in step 221, it is againdetermined whether the state of the amplifier is linearizable. Thisprocess continues until either the amplifier's state is found to belinearizable or the amplifier reaches the same configuration as thatdefined by State 1, in which case the amplifier is placed into State 1and the method is terminated.

Once the method is complete, the amplifier has been optimally configuredfor its current operating conditions. As those conditions change (e.g.,due to variations in temperature, or changes to the types of inputsignal being supplied to the amplifier), the method of FIG. 2 may bere-executed in order to re-calibrate the amplifier according to its newoperating conditions. Once the amplifier is calibrated according to themethod of FIG. 2 (and, specifically, once the adjustable attenuators andphase shifters are calibrated), the amplifier can be put into operation.

Additionally, in some cases, by maintaining the maximum efficiency ofthe Doherty amplifier, care should be taken to circumvent peak signalclipping at the output of the amplifier. Thus by retaining an unclippedoutput, the algorithm locates the state configuration to obtain bestefficiency. Conversely, if the peak power is clipped, the attenuator andphase shifter state is varied to attain better linearity. By sweepingthe attenuation in the peaking amplifier path and measuring the averageoutput power and PAR, the peak power can be calculated. By comparing themeasured peak power with the peak power capability of the amplifier, itis possible to detect any peak signal clipping caused by sweeping theattenuation. As such, if the clipped signal exceeds a threshold level(e.g., 0.2 dB) the attenuation sweep will be stopped.

As described above, States 1, 2, and 3 are boundary conditions thatdefine candidate calibrations for the attenuation and phase states of apower splitter of a Doherty amplifier. To illustrate graphically theboundary conditions defined by States 1, 2, and 3, FIG. 6 is a graphthat shows the relative position of each State. FIG. 6 depicts sampledata that may be captured during the execution of the method depicted inFIG. 2 against a nominal Doherty amplifier. The graph's Y-axis showsamplifier efficiency, while the X-axis shows phase shift of theamplifier (i.e., the relative phase shift between the carrier andpeaking paths). FIG. 6 depicts a number of lines that show amplifierefficiency at a number of different attenuation values as a function ofphase shift. Accordingly, in FIG. 6, each line represents the efficiencyof the amplifier at a particular attenuation state as a function ofphase shift. In FIG. 6 the attenuation values refer to attenuation alongthe peaking amplifier path (e.g., the attenuation of the carrieramplifier path is fixed to 0 dB).

Because each one of States 1, 2, and 3, are defined by a particularattenuation and phase state of the power splitter of the Dohertyamplifier, each State can be mapped onto one of the lines of FIG. 6.Therefore, a number of points have been identified on FIG. 6, where eachpoint is associated with one of State 1, State 2, or State 3, definedabove with reference to FIG. 2. Point 615 illustrates State 1 for theamplifier. State 1 defines the phase shift of the amplifier that, atequal attenuation on each amplifier path, results in the amplifier'speak power. Point 615, therefore, demonstrates the phaseshift/attenuation setting for the amplifier that defines State 1. Point617 illustrates State 2 for the amplifier. State 2 defines the phaseshift of the amplifier that, at equal attenuation on each amplifierpath, results in the amplifier's highest efficiency. Point 617,therefore, demonstrates the phase shift/attenuation setting for theamplifier that defines State 2. Point 619 illustrates State 3 for theamplifier. State 3 defines the attenuation level of the amplifier that,at the phase shift defined by State 2, results in the amplifier'shighest efficiency. Point 619, therefore, demonstrates the phaseshift/attenuation setting for the amplifier that defines State 3.

The three points 615, 617 and 619, as illustrated in FIG. 6, define aspace 621 (i.e., a triangle) within which fall a number of attenuationand phase states of the amplifier. Each state that falls within space621 is a candidate optimal setting for the amplifier. In general, it isdesirable to configure the amplifier in the most efficient state (e.g.,the state closest to point 619 that falls within space 621) that stillprovides acceptable linearity. Therefore, as long as the linearizationsystem of amplifier can correct the nonlinearity of the amplifier withinthe measured three states, the attenuator and phase shifter state istuned towards best efficiency state identified by point 619. On thecontrary, if the linearization system is not able to correctnonlinearity the attenuator and phase shifter values of the amplifier'spower splitter are shifted towards the state having better linearity(e.g., towards the state defined by point 615).

Accordingly, if the attenuation and phase states associated with State 3will result in an amplifier calibration that is not linearizable, theamplifier is adjusted towards State 1 (point 615). For example, if theamplifier is not linearizable when configured in accordance with point619, the amplifier may be configured in accordance with the attenuationand state of point 623 (or the closest configuration possible to point623 that falls within space 621). Then, if the amplifier is notlinearizable when configured in accordance with point 623, the amplifiermay be configured in accordance with the attenuation and state of point625 (or the closest configuration possible to point 625 that fallswithin space 621), and so on.

The present system and method, therefore, may avoid the complexity ofmeasuring all possible combinations of attenuator and phase shifter of apower splitter of a Doherty amplifier for attaining an optimum RFperformance. By identifying States 1, 2, and 3, as described above, andselecting a state falling with an area define by those boundary states,the amplifier can quickly be configured for optimal performance.Additionally, embodiments of the present system and method can beimplemented using a reduced number of standard measurement instruments,which are cost effective and widely available.

It will be appreciated that the above described system and method forcalibrating and adjustable signal or power splitter operating inconjunction with a Doherty amplifier may be implemented with one or moreintegrated circuits or hybrid structures or combinations or the like.

An embodiment of a method of calibrating an amplifier having a firstamplification path and a second amplification path includes setting anattenuation of the first amplification path to a first attenuation valueand an attenuation of the second amplification path to the firstattenuation value, determining a first phase shift of the firstamplification path and a second phase shift of the second amplificationpath that meets a first performance criteria, setting a phase shift ofthe first amplification path to the first phase shift and a phase shiftof the second amplification path to the second phase shift, anddetermining a first attenuation of the first amplification path and asecond attenuation of the second amplification path that meets a secondperformance criteria.

A method of calibrating a power splitter connected to an amplifier, theamplifier having a carrier path and a peaking path includes supplying aninput signal to the power splitter. The power splitter is configured todetermine an attenuation and a phase shift of the carrier path and anattenuation and a phase shift of the peaking path. The method includesidentifying a first state of the power splitter defining a first phaseshift of the carrier path and a second phase shift of the peaking pathproviding a maximum efficiency of the amplifier when the attenuation ofthe carrier path and the attenuation of the peaking path are set tofirst attenuation values, and setting the attenuation of the carrierpath and the attenuation of the peaking path to achieve a maximumefficiency of the amplifier when the phase shift of the carrier path andthe phase shift of the peaking path are set according to the firststate.

A system for calibrating and amplifier includes an amplifier having afirst path and a second path, a power splitter, and a controller. Thepower splitter is coupled to the amplifier, and the power splitterincludes a first adjustable attenuator connected to the first path, asecond adjustable attenuator connected to the second path, a firstadjustable phase shifter connected to the first path, and a secondadjustable phase shifter connected to the second path. The controller iscoupled to the first and second adjustable attenuators and the first andsecond adjustable phase shifters. The controller is configured to set anattenuation of the first path to a first attenuation value and anattenuation of the second path to the first attenuation value, determinea first phase shift of the first path and a second phase shift of thesecond path providing a maximum efficiency of the amplifier, set a phaseshift of the first path to the first phase shift and a phase shift ofthe second path to the second phase shift, and determine a firstattenuation of the first path and a second attenuation of the secondpath providing a maximum efficiency of the amplifier.

This disclosure is intended to explain how to fashion and use variousembodiments in accordance with the invention rather than to limit thetrue, intended, and fair scope and spirit thereof. The foregoingdescription is not intended to be exhaustive or to limit the inventionto the precise form disclosed. Modifications or variations are possiblein light of the above teachings. The embodiment(s) was chosen anddescribed to provide the best illustration of the principles of theinvention and its practical application, and to enable one of ordinaryskill in the art to utilize the invention in various embodiments andwith various modifications as are suited to the particular usecontemplated. All such modifications and variations are within the scopeof the invention as determined by the appended claims, as may be amendedduring the pendency of this application for patent, and all equivalentsthereof, when interpreted in accordance with the breadth to which theyare fairly, legally, and equitably entitled.

What is claimed is:
 1. A method of calibrating an amplifier having a first path and a second path, the method comprising: identifying a first state of the amplifier defining a first phase shift of the first path and a second phase shift of the second path resulting in a maximum efficiency of the amplifier when an attenuation of the first path and an attenuation of the second path are set to first attenuation values; and setting the attenuation of the first path and the attenuation of the second path to achieve a maximum efficiency of the amplifier when the phase shift of the first path and the phase shift of the second path are set according to the first state.
 2. The method of claim 1, wherein identifying the first state of the amplifier includes sweeping the phase shift of the first path and the phase shift of the second path through a number of values.
 3. The method of claim 1, wherein the first attenuation values are 0 dB on the first path and 0 dB on the second path.
 4. The method of claim 1, wherein the first attenuation values are equal.
 5. The method of claim 1, including: determining whether the amplifier is linearizable; and when the amplifier is not linearizable, adjusting either or both the attenuation or the phase shift of either or both the first path or the second path.
 6. The method of claim 5, wherein adjusting the attenuation or the phase shift of the first path or the second path includes: identifying a second state of the amplifier defining a third phase shift of the first path and a fourth phase shift of the second path resulting in a maximum peak output power of the amplifier when the attenuation of the first path and the attenuation of the second path are set to the first attenuation values; and adjusting the attenuation and the phase shift of the first path and the attenuation and the phase shift of the second path towards the second state.
 7. The method of claim 6, wherein identifying the second state of the amplifier includes sweeping the attenuation of the first path or the attenuation of the second path through a number of values.
 8. A system, comprising: an amplifier having a first path and a second path; a controller coupled to the first path and the second path, the controller being configured to: identify a first state of the amplifier defining a first phase shift of the first path and a second phase shift of the second path resulting in a maximum efficiency of the amplifier when an attenuation of the first path and an attenuation of the second path are set to first attenuation values; and set the attenuation of the first path and the attenuation of the second path to achieve a maximum efficiency of the amplifier when the phase shift of the first path and the phase shift of the second path are set according to the first state.
 9. The system of claim 8, wherein the controller is configured to identify the first state of the amplifier by sweeping the phase shift of the first path and the phase shift of the second path through a number of values.
 10. The system of claim 8, wherein the first attenuation values are 0 dB on the first path and 0 dB on the second path.
 11. The system of claim 8, wherein the first attenuation values are equal.
 12. The system of claim 8, wherein the controller is configured to: determine whether the amplifier is linearizable; and when the amplifier is not linearizable, adjust either or both the attenuation or the phase shift of either or both the first path or the second path.
 13. The system of claim 12, wherein the controller is configured to adjust the attenuation or the phase shift of the first path or the second path by: identifying a second state of the amplifier defining a third phase shift of the first path and a fourth phase shift of the second path resulting in a maximum peak output power of the amplifier when the attenuation of the first path and the attenuation of the second path are set to the first attenuation values; and adjusting the attenuation and the phase shift of the first path and the attenuation and the phase shift of the second path towards the second state.
 14. The system of claim 13, wherein identifying the second state of the amplifier includes sweeping the attenuation of the first path or the attenuation of the second path through a number of values.
 15. An amplifier including a first path and a second path and a power splitter coupled to the amplifier, the power splitter including a first adjustable attenuator connected to the first path, a second adjustable attenuator connected to the second path, a first adjustable phase shifter connected to the first path, and a second adjustable phase shifter connected to the second path, the amplifier having been configured into an optimized state by a process of: identifying a first state of the amplifier defining a first phase shift of the first path and a second phase shift of the second path resulting in a maximum efficiency of the amplifier when an attenuation of the first path and an attenuation of the second path are set to first attenuation values; and setting the attenuation of the first path and the attenuation of the second path to achieve a maximum efficiency of the amplifier when the phase shift of the first path and the phase shift of the second path are set according to the first state.
 16. The amplifier of claim 15, wherein identifying the first state of the amplifier includes sweeping the phase shift of the first path and the phase shift of the second path through a number of values.
 17. The amplifier of claim 15, wherein the first attenuation values are 0 dB on the first path and 0 dB on the second path.
 18. The amplifier of claim 15, wherein the first attenuation values are equal.
 19. The amplifier of claim 15, wherein the process includes: determining whether the amplifier is linearizable; and when the amplifier is not linearizable, adjusting either or both the attenuation or the phase shift of either or both the first path or the second path.
 20. The amplifier of claim 19, wherein adjusting the attenuation or the phase shift of the first path or the second path includes: identifying a second state of the amplifier defining a third phase shift of the first path and a fourth phase shift of the second path resulting in a maximum peak output power of the amplifier when the attenuation of the first path and the attenuation of the second path are set to the first attenuation values; and adjusting the attenuation and the phase shift of the first path and the attenuation and the phase shift of the second path towards the second state. 